Assistive properties are properties that QuickTest learns only if the mandatory properties that QuickTest learns for a particular object in your application are not sufficient to create a unique description. If several assistive properties are defined for an object class, then QuickTest learns one assistive property at a time and stops as soon as it creates a unique description for the object. If QuickTest does learn assistive properties, those properties are added to the test object description.
[Taken from QTP User Guide]